Manufacture of carbon disulfide



Patented Apr. 28, 1953 UNITED STATES MANUFACTURE OF CARBON DISULFIDE poration of Delaware No Drawing. Application December. 15., 1947, Serial No. 791,937

G'Claims. (Cl. 223-206) This invention relates to a. method for converting, mixtures, of hydrocarbons into useful sulfur compounds, and more particularly it relates to animproved method for converting natural gas rich in methane to carbon disulfide. In application Serial Number 754,776, filed June 14, 1947, in the. names of I-lillis O. Folkins and Elmer Miller as co-inventors there is disclosed and claimed a method for eliminating carbon formation and side reactions during the conversion of hydrocarbongas mixtures to carbon disulfide by catalytic reaction with sulfur by controlling the C3 and higher molecular weight hydrocarbon content of the gas so as not to exceed about 4 mole per cent.

In United States Patent 2,330,934, of October 5, 1943, to 'Carlisle M. Thacker, assigned to The Pure Oil Company, there is disclosed a method of converting hydrocarbons into sulfur compounds, Which involves contacting a mixture of the hydrocarbon gas and sulfur vapor at an elevated temperature with a catalyst such as silica gel, bauxite, certain catalytic clays and activated alumina. It has been found in operation of the process that where natural gases are used which contain any substantial amount of higher hydrocarbons, such as C2, C3, C4, C5, and Cs hydrocarbons, but particularly C3 and higher molecular weight hydrocarbons, certain side reactions occur which result in the formation of coke and viscous polymeric carbon-sulfur compounds which contaminate the catalyst mass and reduce the efie ciency of the process to a material degree. These side reactions origin-ate with the heavier hydrocarbon components of natural gas at operating conditions adequate forhigh conversions to carbon disulfide of the lighter hydrocarbons, which comprise the greater fraction of a natural gas charge. Moreover, heavy carbon-sulfur compounds of this type contaminate the unconverted sulfur to such an extent that its recycling in the process is complicated by expensive purification steps. Coke-like materials are also produced by the decomposition of these polymeric compounds or by the destructive reaction of the heavier hydrocarbons in the charge gas with sulfur.

Accordingly, it is an object of this invention to improve the efficiency of processes for the conversion of hydrocarbon to organic sulfur compounds and, in particular, the process for converting natural hydrocarbon gases to carbon disulfide.

It is a second object of the invention toprovide an, effective of. improving the sulfur con.- versionand simplifyingthe sulfur recovery from processes for the synthesis of organic sulfur compounds from hydrocarbons.

It is a third object of the invention to provide an improved process for the conversion of hydrocarbons to organic sulfur compounds in which the life of the catalyst used in the conversion is substantially prolonged.

It is a fourth object of the invention to provide a method for improved operation of high temperature hydrocarbon conversion processes involving reaction with sulfur whereby the formation of coke in the hydrocarbon processing stages is substantially avoided.

Other objects and advantages of the invention will in part be obvious and in part appear hereinafter.

I have discovered that the conversion of natural gas to carbon disulfide at temperatures above 500 (3., is materially improved if the proportion of the heavier constituents in the natural gas is reduced by a selective cracking process which converts the heavier ends of the natural gas mixture to lighter C1, C2 and C3 hydrocarbons, so that the content of the C3 and higher molecular weight hydrocarbons is lowered to less than about e per cent of the total gas mixture. In accordance with good practice for carrying out the process, the ratio of the proportion of methane and C2 hydrocarbon to the heavier gases in the hydrocarbon mixture should be at least about 24, with the C2 hydrocarbon present in amount not more than about 10 per cent of the total mixture. The invention thus comprises the several steps constituting the selective cracking process for the preparation of natural gas for use in the synthesis of sulfur compounds from natural gas and relating the adjusted composition of the gas to each of the steps thereof, which process will be exemplified in the specific embodiments hereinafter disclosed, and the scope of the invention will be indicated in the claims.

Typical natural gas will contain by volume about 90 per cent of methane, about 5 per cent of C2 hydrocarbon, about 2.5 per cent of C3 hydrocarbon, 1.5 per cent of C4, and fractions of 1.0 per cent of C5 and higher hydrocarbons. The use of such a natural gas as it occurs or in about that composition in the synthesis of carbon disulfide results in inducing side reactions which consume a material proportion of the sulfur fed to the process as complex polymeric organic sulfur compounds. Such side reactions also reduce the efifectiveness of the catalyst by depositing the polymeric compounds and coke on the catalyst and throughout the catalyst chamber. If the ratio of the, proportion of the several gases in the mixture is altered by subjecting the natural gas to a selective cracking process, a substantially improved operation is developed which results in a clean and efiicient conversion of the sulfur to desired compounds. An empirical relationship which indicates an approximate dividing line between an efficiently operating process and an inefliciently operating process can be stated as a light to heavy hydrocarbon ratio, defined in the terms of the following formula:

in which C1, C2, etc., represent the mole percentages of the indicated hydrocarbons in the mixture. This ratio is merely a first approximation of a useful range for the gas composition. It is apparent that it emphasizes'a predominance of C1 and C2 hydrocarbons as compared with a minute amount of the heavier hydrocarbons. Hydrocarbons containing 3 carbon atoms are rather transitional in their effect, and if present in amount exceeding about 4 per cent, will adversely affect the reaction. Heavier hydrocarbons containing 4 or more carbon atoms should not exceed a total of about 1 per cent, C4 being limited to about 1 per cent or less if hydrocarbons higher than C4 are not present, or to about 0.7 per cent if small amounts of C5 or C'e hydrocarbons are present, and C5 and heavier hydrocarbons to less than 0.3 per cent. Thus, the maximum proportion of C3 and higher hydrocarbons which should be allowed in the feed gas is about 4 per cent and the light to heavy hydrocarbon ratio is maintained at about 24 or higher. Hydrocarbons of the C4 range and higher homologues are most conducive to the occurrence of side reactions when they are present in appreciable amounts in a natural gas to be processed. Thus, if C4 and higher hydrocarbons are substantially removed, the maximum amount of C3 allowable is proportionately increased.

Relatively mild conditions will induce sufficient thermal cracking to correct the composition of the gas to the desired range. In general, thermal cracking conditions of temperature and pressure approximating the temperature and pressure conditions of the hydrocarbon-sulfur reaction will be found satisfactory. Typical conditions which bring about sufiicient cracking of the heavier constituents of the gas involve subjecting it in thermal cracking apparatus to temperatures in the range from about 600 C. to about 800 C. at atmospheric or superatmospheric pressures. The residence time of the gas in the apparatus should be sufficient to bring about the desired cracking,

and it will be found to be about 5 to about 60 seconds.

Of course, catalytic cracking operations can be used to prepare the gas, for the particular means used to crack the gas would not affect its usefulness for subsequent processing.

Catalysts which can be used in the carbon disulfide forming step include such substances as synthetic silica-alumina catalysts, particularly those containing from 2 to per cent by weight of silica, silica gel, fullers earth, bauxite, activated alumina and in general those types of clay which are effective in the removal of color bodies and gum forming bodies from petroleum oils. The catalysts may be used alone or in combination with one or more compounds of metals of Groups V, VI, VII, and VIII of the periodic table as promoters. The oxides and sulfides of such metals are effective promoters in the type of reaction contemplated. For example, oxides and sulfides of iron, vanadium, chromium, molybde- 4 num, and manganese can be used as promoters in combination with silica gel, fullers earth or activated alumina catalysts. The temperature for the process is preferably Within 500 to 700 C., an the process is carried out by injecting the cracked hydrocarbon gas at or above reaction temperature into the contact zone wherein the hydrocarbon gas meets hydrogen sulfide, sulfur Vapor or a sulfur compound capable of giving hydrogen sulfide or sulfur vapor at the temperature of reaction. Details of the operation of the process can be found in such prior disclosures as United States Patent 2,330,934, cited, and United States Patent 2,309,718, of February 2, 1943, to Carlisle M. Thacker, assigned to The Pure Oil Company.

The following typical examples will illustrate a few variants of the process:

Example 1.A natural gas containing 92.7 per cent methane, 3.8 per cent ethane, 1.8 per cent propane, 0.9 per cent of C4 hydrocarbon, 0.6 per cent of C5 hydrocarbon and 0.2 per cent of Ce hydrocarbons is passed through a conventional tubular thermal cracking furnace, maintained at a temperature of 700 C. and atmospheric pres sure at such a rate that the residence time of the gas in the furnace at that temperature is 36 seconds. The product issuing from the furnace has substantially the following composition: hydrogen 0.7 per cent, methane 92 per cent, ethane 5.8 per cent, propane 1.0 per cent, C4 hydrocarbons 0.3 per cent and C5 and heavier hydrocarbons 0.2 per cent. When this hot gas issuing from the cracking furnace is passed at substantially atmospheric pressure into a reaction zone maintained at a temperature of about 600 C., and fitted with a static bed of silica gel catalyst, efficient conversion thereof to carbon disulfide is obtained when a stoichiometric ratio of the gas to sulfur charge is at a space velocity of about 634. When the gas is thus freed of the heavier constituents and the composition balanced in favor of lighter gases, competing side reactions which normally occur in the conversion of sulfur are reduced to such an extent that there is substantiall no accumulation of sulfur polymers in the catalyst bed or receiving end of the apparatus.

Example 2.A natural gas containing 92.7 per cent methane, 3.3 per cent ethane, 1.8 per cent propane, 0.9 per cent of C4 hydrocarbons, 0.6 per cent of C5 hydrocarbons and 0.2 per cent of Ce hydrocarbons is passed through a conventional tubular thermal cracking furnace, maintained at a temperature of 650 C., at such a rate that the residence time of the gas in the furnace at that temperature is 12 seconds. The product issuing from the furnace has substantially the following composition: hydrogen 0.2 per cent, methane 92.6 per cent, ethane 4.7 per cent, propane 1.7 per cent, C4 hydrocarbons 0.5 per cent and C5 and heavier hydrocarbons 0.3 per cent. This treated gas is likewise used for conversion to carbon disulfide in a process like that described in connection with Example 1. Under a similar set of reaction conditions, it was found that side reactions were substantially eliminated and that conversion of the hydrocarbon gas and the conversion of sulfur to carbon disulfide was materially improved as evidenced by a reduction of the amount of coke formed in the furnace and a concurrent reduction in the amount of complex sulfur compound formed.

A comparative test using a natural gas without preliminary preparation was made with the following results:

Example 3.A natural gas containing 91 per cent methane, 5 per cent ethane, 2 per cent propane, 1 per cent of C4 hydrocarbons, 0.5 per cent of pentanes and 0.5 per cent hexanes and heavier hydrocarbons was passed at substantially atmospheric pressure into a reaction zone maintained at a temperature of 600 C. and fitted with a static bed of silica gel catalyst. Employing a stoichiometric ratio of gas and sulfur at a total space velocity of 634 (gas and sulfur (S2) volume calculated at C. and 760 millimeters of mercury), a conversion of 47 per cent of the hydrocarbon gas to carbon disulfide was obtained. It was found under these conditions that competing side reactions occurred to such an extent that about 2 per cent of the charged gas reacted with the sulfur to yield a viscous tarry polymeric material and some coke, with the result that a material decrease in over-all efficiency and catalytic activity followed. Recovered sulfur was 53 per cent of that charged. An initial high conversion of around 67 per cent was attained for a period of about one hour after which conversion dropped gradually and after about six hours operation, conversion leveled off at about 47 per cent.

The pressure to which the natural gas is cracked should be substantially equal to the pressure used in the carbon disulfid synthesis to facilitate feeding the two simultaneously to a reaction chamber. Normally, as long as contact time of gas and catalyst is held constant, the pressure in the cracking chamber will not have any effect on the cracking of the natural gas in that pressure range where the carbon disulfide process is usually operative. It is understood that increasing the pressure on the cracking chamber will increase the contact time of gases therein, but this is a controllable variable and can be adjusted so that a constant contact time is achieved either by increasing the throughput of natural gas, or by reducing the size of the cracking furnace.

Although the examples have dealt with the adjustment of natural mixtures of hydrocarbons for processing, other gases which can be brought to a, desirable light to heavy hydrocarbon ratio can be used. Thus, gases obtained from cracked petroleum oils, by-product coke oven gases and other gases containing a proportion of light hydrocarbons which would justify concentration to a useful range for the instant process can be used.

Generally it will be found that, regardless of the source of the gas, alteration of its composition in favor of light hydrocarbons will improve operation. When the selective preliminary cracking operation described is employed, it will be found that residence times in the range from about 5 to about .60 seconds at temperatures and pressures matching the needs of the sulfur conversion process will be adequate.

In this specification, the term space velocity" means the number of volumes of gas reduced to standard conditions passing through th catalyst per hour compared to the volume of the catalyst.

While this process is directed chiefly to the adjustment of natural gas, rich in methane, to compositions suitable for use as charge to catalytically react with sulfur for the production of carbon disulfide and hydrogen sulfide, it is also possible to prepare or purify other mixed hydrocarbon stocks so that minor components of such gases with undesirably high chemical reactivity with sulfur, under specified conditions, may be substantially removed. Thus, a gas composed substantially of propane, but containing minor proportions of heavier hydrocarbons may be treated to reduce the proportion of these heavier hydrocarbons and thus prepare a charge stock of more uniform reactivity for the production of carbon disulfide.

What is claimed is:

1. In the method of converting natural gas containing in excess of 1 mole per cent of C4 and higher molecular weight hydrocarbons to carbon disulfide by reaction with sulfur in the presence of catalysts capable of promoting the formation of carbon disulfide therefrom at temperatures of approximately 500 to C., the improvement which comprises preparing the natural gas for reaction by subjecting it to thermal cracking under temperature-time conditions within the range of about 600 to 800 C. and 5 to 60 seconds to reduce the C3 and higher molecular weight hydrocarbon content to less than 4 mole per cent and the C4 and higher molecular weight hydrocarbon content to less than 1 mole per cent of the gas prior to reacting it with the sulfur in the presence of the catalyst, thereby to substantially completely eliminate the formation of tarry material in the catalyst and resulting fouling thereof.

2. The method in accordance with claim 1 in which the C4 hydrocarbons in the natural gas are reduced to not more than 0.7 mole per cent and the C5 and higher hydrocarbons are reduced to not more than 0.3 mole per cent.

3. The method in accordance with claim 1 in which C4 and higher molecular weight hydrocarbon content of the gas is reduced to about 0.8 mole per cent, the sulfur to hydrocarbon content of the reaction mixture is about 10% in excess of the stoichiometric amount necessary to produce carbon disulfide and the reaction temperature is about 600 C.

4. The method in accordance with claim 1 in which the C4 and higher molecular weight hydrocarbon content of the natural gas is reduced to about 0.5 mole per cent prior to reacting it with the sulfur.

5. The method in accordance with claim 4 in which the relationship between hydrocarbons and sulfur charged to the reaction zone is approximately stoichiometric.

6. The method in accordance with claim 5 in which the cracking operation is conducted at substantially atmospheric pressure.

MILTON M. MARISIC.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Thacker Oct. 5, 1943 OTHER REFERENCES Number 

1. IN THE METHOD OF CONVERTING NATURAL GAS CONTAINING IN EXCESS OF 1 MOLE PER CENT OF C4 AND HIGHER MOLECULAR WEIGHT HYDROCARBONS TO CARBON DISULFIDE BY REACTION WITH SULFUR IN THE PRESENCE OF CATALYSTS CAPABLE OF PROMOTING THE FORMATION OF CARBON DISULFIDE THEREFROM AT TEMPERATURES OF APPROXIMATELY 500* TO 700*C. THE IMPROVEMENT WHICH COMPRISES PREPARING THE NATURAL GAS FOR REACTION BY SUBJECTING IT TO THERMAL CRACKING UNDER TEMPERATURE-TIME CONDITIONS WITHIN THE RANGE OF ABOUT 600* TO 800*C. AND 5 TO 60 SECONDS TO REDUCE THE C3 AND HIGHER MOLECULAR WEIGHT HYDROCARBON CONTENT TO LESS THAN 4 MOLE PER CENT AND THE C4 AND HIGHER MOLECULAR WEIGHT HYDROCARBON CONTENT TO LESS THAN 1 MOLE PER CENT OF THE GAS PRIOR THE REACTING IT WITH THE SULFUR IN THE PRESENCE OF THE CATALYST, THEREBY TO SUBSTANTIALLY COMPLETELY ELIMINATE THE FORMATION OF TARRY MATERIAL IN THE CATALYST AND RESULTING FOULING THEREOF. 